1. Field of the Invention
The present invention relates to a pump apparatus, which is capable of always discharging a constant amount of a fluid using a piston that is displaceable in accordance with a pilot pressure.
2. Description of the Related Art
A metering discharge pump has been used for supplying a constant amount of a chemical liquid, paint, washing liquid or the like, for example, in an apparatus for producing a semiconductor or the like, a coating apparatus, and a medical apparatus.
A bellows type pump is used as such a metering discharge pump in many cases, wherein suction and discharge pressures are obtained such that an accordion-shaped bellows, which is installed to surround a drive shaft, is expanded and contracted under the driving action of a motor or the like.
A metering discharge pump according to a conventional technique is disclosed, for example, in Japanese Laid-Open Patent Publication No. 10-47234, wherein a valve housing and a pump housing, in which a first valve unit and a second valve unit are arranged respectively, are provided in an integrated manner.
The metering discharge pump disclosed in Japanese Laid-Open Patent Publication No. 10-47234 is designed such that a drive shaft is displaced in an axial direction under the driving action of a motor, and a forward end of a bellows, which is installed at the forward end of the drive shaft, is displaced within a pump chamber formed in the pump housing. The accordion-shaped bellows, which is arranged in the pump chamber, undergoes a linear reciprocating displacement integrally with the drive shaft, whereby the bellows expands and contracts.
More specifically, a construction is adopted in which suction pressure is generated by contracting the bellows inside the pump chamber, and liquid is sucked from the outside in order to fill the interior of the pump chamber with a predetermined amount of liquid. On the other hand, a discharge pressure is generated by expanding the bellows inside the pump chamber under the displacement action of the drive shaft, and thus liquid is discharged from the pump chamber to the outside.
When the metering discharge pump according to such a conventional technique is used, however, it is feared that pulsations may occur within the fluid, as a result of the expanding and contracting actions of the bellows, when the fluid is discharged from the pump chamber to the outside.
Further, in the industrial field of semiconductor production apparatus and the like, in view of the high cost of the coating liquid (resist solution), it is essential that the flow rate of the fluid be controlled highly accurately when the fluid is discharged.